Methods for forming a desired image by developing an electrostatic latent image with a toner, have been widely used.
For example, in electrophotography, an electrostatic latent image formed on a photoconductor is developed with a toner made from colored particles optionally containing other particles of an external additive, carrier, etc. Then, the developed image is transferred onto a recording medium such as a paper or OHP sheet and then fixed to obtain a printed product.
There is an increasing need for color image forming devices using electrophotography, such as a color copying machine, a color facsimile machine and a color printer. In the formation of color images by full-color electrophotography, colors are reproduced with color toners in three colors, which are generally yellow, magenta and cyan, or with color toners in a total of four colors, which are the three colors and black. In the case of color copying, an image is formed by the following method, for example: first, a color image is read and decomposed into pixels and then converted to digital image signals in different colors; light is applied onto a charged photoconductor to form an electrostatic latent image; then, the image is developed on the photoconductor, using color toners that correspond to electrostatic latent image signals in different colors; finally, the image is transferred onto a recording medium such as a paper or OHP sheet.
In general, methods for producing toners used for development are broadly classified into a pulverization method and a polymerization method.
In the pulverization method, polymer particles are produced by pulverizing and classifying a solid colored resin product obtained by melt-kneading a binder resin and a colorant.
The polymerization method is a method for producing polymer particles by forming and polymerizing droplets of a polymerizable monomer composition containing a polymerizable monomer and a colorant. While the form of the polymer particles obtained by the pulverization method is not uniform, the form of the polymer particles obtained by the polymerization method is close to a spherical form, and the particles have a small particle diameter and a narrow particle diameter distribution. Especially from the viewpoint of improving image properties such as image reproducibility and fineness, toners with a highly-controlled form and particle diameter distribution, like toners produced by the polymerization method (i.e., polymerized toner), have been increasingly used.
Various kinds of properties are required of toners, such as environmental stability from the viewpoint of preventing image deterioration due to changes in temperature, humidity, etc., printing durability from the viewpoint of reducing toner consumption, and low-temperature fixability from the viewpoint of reducing power consumption.
In the case of toners for developing electrostatic images, toner particles containing polymer particles and an external additive attached thereto are charged and then supplied onto a photoconductor having an electrostatic latent image. Or, the toner particles and a member such as a developing blade are charged in between and then supplied onto a photoconductor having an electrostatic latent image, or the toner particles and a carrier are charged in between and then supplied onto a photoconductor having an electrostatic latent image. In this supplying step, the toner in an amount which corresponds to the charge density of the electrostatic latent image is attached onto the photoconductor. A high-quality image is formed when the toner is appropriately charged.
However, once a decrease or non-uniformity in toner charge amount is caused by environmental changes such as temperature change or humidity change, a desired electrostatic latent image cannot be developed on a photoconductor and results in problems such as fog, unevenness of images, and changes in image density.
To prevent variations in toner charge amount due to environmental changes, various kinds of additives have been studied.
Patent Literature 1 discloses a method for producing a toner for developing electrostatic images, which contains 80 to 500 ppm of a cyano group-containing hydrocarbon compound and a positively-chargeable charge control resin or agent. Also, Patent Literature 1 mentions that the toner is poor in heat-resistant storage stability and printing durability when the cyano group-containing hydrocarbon compound is more than 500 ppm, and the charge stability of the toner deteriorates when the cyano group-containing hydrocarbon compound is less than 80 ppm. However, the toner disclosed in patent Literature 1 has a problem in that a slight amount of the cyano group-containing hydrocarbon compound volatilizes when the toner is fixed.